Making and testing ammonia

Read our standard health & safety guidance

Lesson organisation

Because ammonia has a strong smell and is poisonous in quantity, pupils should, ideally, carry out this experiment in fume cupboards. Alternatively, provided that the quantities of reactants are no greater than as stated here, the experiment could be carried out in a well-ventilated laboratory.

Depending on arrangements for setting up and clearing away, the experiments should take around 30 minutes.

Apparatus and Chemicals

Eye protection

Each working group will require:

Retort stand, boss and clamp
Boiling tube with stopper and delivery tube – see diagram. This apparatus must be dry (see note 1)
Boiling tubes, 2 – must be thoroughly dry (see note 1)
Beaker (100 cm³)
Beaker (250 cm³) or larger
Bunsen burner
Heat resistant mat
Spatula

Students will need access to:

Ammonium chloride (Harmful) (see note 3)
Calcium hydroxide (Irritant) (see note 3)
Calcium oxide (Irritant) - (optional) (see note 3)
Concentrated hydrochloric acid (Corrosive) (see note 2)

Red Litmus paper
Blue Litmus paper
Universal indicator paper

Technical notes

Ammonia gas (Toxic, Dangerous for the environment) Refer to CLEAPSS Hazcard 5
Ammonium chloride (Harmful) Refer to CLEAPSS Hazcard 9A
Calcium hydroxide (Irritant) Refer to CLEAPSS Hazcard 18
Calcium oxide (Irritant) Refer to CLEAPSS Hazcard 18
Concentrated hydrochloric acid (Corrosive) Refer to CLEAPSS Hazcard 47A

1 Ammonia (Toxic, Dangerous for the environment) is a very soluble gas. If the preparation and collection apparatus is not dry, the solubility experiment will not work.

2 Two drops of the acid in the bottom of a stoppered, labelled test-tube are sufficient. A small number of prepared tubes could be arranged around the laboratory so that students can access them only under supervision.

3 Small quantities of the solid chemicals should be provided in stoppered bottles. Use lumps or granules of calcium oxide (optional).

Procedure

HEALTH & SAFETY: Eye protection must be worn at all times. Ammonia gas is Toxic and Dangerous for the environment and pungent-smelling and must not be inhaled. The experiments must only be carried out in a fume cupboard or in a well-ventilated laboratory.

a In a small beaker, mix 2 spatulas of the ammonium chloride with 2 spatulas of the calcium hydroxide together. The two solids begin to react immediately on mixing.

b Hold a piece of each colour of Litmus paper over the mixture and observe the colour change. Test also with a piece of Universal indicator paper.

c Transfer the mixture of ammonium chloride and calcium hydroxide into a boiling tube and set up the apparatus as shown in the diagram.

d Optional step - put the lump of calcium oxide into the boiling tube containing the ammonium chloride/calcium hydroxide mixture. The calcium oxide will absorb the water produced in the reaction and ensure that the ammonia gas is dry.

e Gently warm the reaction mixture.

f Collect a test-tube which contains a few drops of concentrated hydrochloric acid. Remove the stopper from this test-tube and hold the open end near the end of the ammonia gas delivery tube. Observe what happens. Replace the stopper on the test-tube of hydrochloric acid and return the test-tube to its original place.

g Two-thirds fill a large beaker with water. This is needed for step j.

h Continue to gently warm the reaction mixture. Hold one of the dry boiling tubes in position as shown in the second diagram. Notice that the ammonia is collected with the boiling tube upside down. This is because ammonia is less dense than air.

i Test around the open end of the collecting boiling tube with Universal Indicator paper to check that the collecting tube is full of ammonia.

j Hold the tube of ammonia upside down then quickly put it, mouth still downwards, into water in a beaker. The ammonia dissolves in the water and the level of the water should rise up inside the test tube. If you want to try this a second time, use a fresh dry boiling tube.

Teaching notes

Ammonia is a very soluble gas. 1 cm³ of water dissolves about 800 cm³ of ammonia at room temperature. A few drops of water will easily dissolve a test-tube of ammonia. This is why it is essential that the apparatus is dry.

The reaction to produce ammonia also produces water. The purpose of the calcium oxide is to help to prevent this water from coming out of the delivery tube as water vapour.

The equation for the generation of ammonia is:

2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2NH₃(g) + 2H₂O(g)

Calcium oxide reacts with water to produce calcium hydroxide:

CaO(s) + H₂O(l) → Ca(OH)₂(s)

Ammonia is one of the very few common alkaline gases. When it dissolves in water it reacts reversibly according to the equation:

NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH^–(aq)

There is generally enough water on the surface of indicator papers to dissolve ammonia without having to moisten the paper.

The white fumes ('smoke') produced with the hydrogen chloride given off by the concentrated hydrochloric acid consist of fine particles of solid ammonium chloride.

The equation is
NH₃(g) + HCl(g) → NH₄Cl(s)

Links

As mentioned above, the high solubility of ammonia in water can be more spectacularly demonstrated in the ammonia fountain experiment.

Health and Safety checked, November 2006